Pentode output circuits



Feb. 13, 1934. R A BRADEN 1,947,184

PENTODE OUTPUT CIRCUITS Filed Feb. 20, 1931 2 Sheets-Sheet l I000 /0,000 FRE'GUF/VCY CYCZES Ff SfCO/VO INVENTOR RENE A BR EN 1 ,oO-MW ATTORNEY Feb. 13, 1934. R. A'. BRADEN PENTODE OUTPUT CIRCUITS Filed Feb. 20. 1931 2 Sheets-Sheet 2 m D V. R E N mm R N 0 W n E NN A E R M. WM,

Patented Feb. 13, 1934 1,947,184 rnNroton oU'rPU'r CIRCUITS Rene A. Braden,

Radio Corporation of America,

Bel-aware Merchantville, N. 3., assigncr to a corporation of Application February 20, 1931. Serial No. 517,154

12 Glairns.

My present invention relates to amplifying circuits, and more particularly to methods of and means for, controlling the output of pentode tube output circuits.

As is well known, the pentode electron discharge tube is provided with the usual cathode, control electrode and anode, and two additional electrodes, to wit: a screen grid electrode, and a third control electrode usually connected to the center of the cathode within the tube, and disposed between the screen grid electrode and the anode. lhis type of pentode tube is intended to be used as a power tube, and is usually employed with a loud speaker. The function of the cathode, control electrode and anode elements in the audio pentode tube are the same as in any triode.

The screen grid is utilized to neutralize to a minimum the inter-electrode capacity existing between the control grid and the anode. The third grid in this tube, as stated above, is usually connected to the electrical mid-point of the cathode within the tube, and in this way is maintained at the same potential as the cathode with respect to the other elements in the tube. In the pentode tube, by reason of the third grid being at the same potential as the cathode, it is found that the effect of secondary electron emission from the anode is effectively minimized by the action of the third grid, which is more negative than the anode. This is accomplished'by virtue of the fact that the third grid is seated nearest the anode, and therefore, influence the secondary electrons before they move towards the vicinity of the positively charged screen grid.

However, when the pentode tube is utilized for energizing a loud speaker it has been found necessary to keep the load impedance very low in comparison with the internal anode resistance of the tube. For example, in the case of one tube which was tested, the optimum load resistance was approximately one-tenth of the internal anode resistance of the tube. If the load impedance is larger or smaller than the optimum value, the amount of amplitude distortion, as measured by the relative amount of second harmonic produced, increases.

The impedance of a loud speaker, as is well known, is different at different frequencies. Considering for example, tests made with a dynamic, or moving coil loud speaker, which is typical of 'all such loud speakers, it Was found by actual measurement that the impedance at 1000 cycles is one-third of the impedance at 5000 cycles, and that the impedance at 100 cycles is one-fifth of the 5000 cycle impedance. When connected through a suitable impedance-adjusting transfori er to a pentode tube Whose optimum load is i000 ohms, the loud speaker impedance would be 2400 ohms at 100 cycles; 4000 ohms at 1000 cycles; and 12000 ohms at 5060 cycles. The total im- 50 pedance in the anode circuit would be only approximately ten per cent higher at 5000 cycles than at 100 cycles, and the alternating anode current would be only ten per cent smaller at 5000 than at 100 cycles.

To demonstrate the significance of the last paragraph, it is necessary to make a comparison with a triode tube. A trlode suitable for use with the loud speaker and transformer described above would have an internal plate resistance of 2000 ohms. At 1000 cycles the alternating anode current would be about eighty per cent of that at 100 cycles; and at 5000 cycles the alternating anode current would be only about thirty per cent of the current at 100 cycles.

Loud speakers have been designed to operate with low impedance tubes, and to give a suitable ratio of high frequency to low frequency response when actuated by a current which varies inversely with the frequency as in the case described above. When utilized with a pentode tube, a loud speaker which gives good results with triodes will be found to have excessive high frequency response because of the constant-current characteristic.

Now, I have discovered a method of, and devised means for controlling the output circuit of a pentode tube so that the high frequency response of the output circuit can be varied in any predetermined, desired manner.

It is, therefore, one of the main objects of my present invention to provide a method of, and means for, controlling the output circuit of an audio pentode tube by employing reactance in the screen grid circuit of the pentode tube in a predetermined manner.

Another important object of the present invention is to provide a method of decreasing the high frequency response of the output circuit of a pentode tube utilized in the power stage of an audio 10o amplifier which consists in adjusting the reactance of the screen grid circuit of the tube until apredetermined desired decrease in said response is secured.

Another object of this invention is to provide 105 a power stage for an audio frequency amplifier utilizing a space discharge tube of the pentode type and employing a moving coil loud speaker wherein the performance of the speaker with a triode tube can be very closely matched with the 110 pentode tube, the latter having a predetermined amount of inductive reactance in its screen grid circuit to secure said matching.

Still another object of the invention is to provide an audio frequency amplifier including a power stage utilizing a pentode tube, there being a network in the screen grid circuit of the tube of suitable characteristics whereby a desired amount of reduction of amplification may be introduced at selected frequencies.

Other objects of the invention are to improve generally the efficiency of power stages employ ing pentode tubes, and to particularly provide a pentode power stage which is not only reliable in operation, but can be readily matched with loud speakers so as to obtain the same performance as with tubes of the triode type.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings,

Fig. 1 diagrammatically shows a circuit employing the present invention,

Fig. 2 is a graphic representation of the effect of the invention as shown in Fig. 1,

Fig. 3 diagrammatically shows a modified form of the invention,

Fig. 4 graphically represents the operation of the modification shown in Fig. 3,

Fig. 5 shows typical curves of different characteristics of a pentode output tube employing the modification shown in Fig. 3.

Referring to the accompanying drawings in which like characters of reference indicate the same parts in the different fi ures, there is diagrammatically shown in Fig. l the power stage of an audio frequency amplifier wherein the powor stage utilizes an electron discharge tube 1 of the pentode type, the tube including a usual cathode 2, control grid 3, and anode i. The source B has its positive terminal connected to the an ode through the load impedance 5, the grid 3 of the tube being negatively biased by a source C, the energy to be amplified being impressed between the terminals 6, 6 and conventionally represented by the designation Input. As has been stated heretofore, a pentcde tube of the audio type employs a screen grid '7, shown positively charged by being connected to the positive terminal of the source '5. The tube, also, includes a secondary emission shield grid 8 disposed between the anode 4 and the positively charged screen grid '7.

The third grid, or shield grid, in this type of tube is connected to the center of the cathode 2 within the tube, thus simplifying the manufacture of the tube, it being understood, or" course, that the cathode 2 is heated in any well known fashion for the emission of electrons therefrom. Thus, the shield grid 8 is mainta ed at the same potential as the cathode, i. e., in relation to the other elements in the tube. By reason of the third grid being at the same potential as the oathode, it is found that the secondary electrons (those thrown oif by the anode) are sent back to the anode by the action of the shield grid 8, which is more negative than anode, with the result that secondary electron emission from the anode, while not completely prevented, is made ineffective.

This explains why the shield grid is seated nearest the anode 4, because it thereby influences the secondary electrons before they move to the vicinity of the positively charged screen grid 7. The connection of the shield grid 8 to the center of the cathode is necessary to insure balance, and to establish this grid at the mean voltage of the two extremes of the cathode legs. It will be understood that the screen grid 7 can be connected to any positive point along the source B, and that the shield grid 8 need not necessarily be connected to the electrical mid-point of the cathode 2 within the tube, but can even be so connected outside of the tube.

As stated heretofore, when the load impedance 5 is a loud speaker and gives good results with a power stage employing a triode, it is found that a pentode tube used in place of the triode has excessive high frequency response because of the constant-current characteristic. A method of decreasing the high frequency response to any desired degree is shown in Fig. 1. An inductance L is inserted in series with the screen grid 7 of the tube, the inductance being so chosen that any desired decrease of high frequency response may be secured.

Thus, in Fig. 2 there is shown a series of curves secured by plotting output volts against frequency cycles per second. The different curves show the effect of various values of inductance L on the pentode tube shown in Fig. 1. In securing the curve shown in Fig. 2 for a pentode output tube with different inductances in the screen grid circuit of the tube, the anode circuit load was a pure resistance of 5000 ohms, but a reactive load such as a loud speaker would not have affected the results qualitatively and quantitatively would have had a very small efiect.

It will be observed from the curves in Fig. 2 that the alternating anode current is practically constant up to a certain frequency, and then decreases more and more rapidly as the frequency increases. (While the ordinates in Fig. 2 are output volts across 5000 ohms, it should be understood that since the output volts are proportional to the alternating anode current, the curves could have been plotted this way.)

The current flowing through a dynamic, or moving coil, loud speaker, connected to a lowimpedance triode varies in the same manner with increasing frequency. Therefore, it will be seen that the performance of a moving coil loud speaker with a triode tube can be very closely matched with a pentode tube having the right amount of inductance in its screen grid circuit. The fundamental principles underlying my invention, and the manner of applying the same are suificiently set forth in Figs. 1 and 2 so as to enable those skilled in the art to embody the invention in a practical circuit. However, the following explanation of the operation of the present invention is given, it being clearly understood that the explanation merely theoretical.

Assuming that there is no impedance in the screen circuit when a signal is impressed on the control grid, an alternating current flows in the screen circuit, although the voltage between screen and cathode is constant and equal to the screen bias voltage. Now, if an impedance is insorted in series with the screen grid, the alternating screen grid current develops a voltage across this impedance, and the screen voltage fluctuates with the control grid voltage. The

phase relation of the fluctuations of screen potential is such that when the anode current increases, the change in screen potential opposes the increase in anode current, and when the anode current decreases, the screen potential opposes the decrease, thus reducing the anode current variation corresponding to a given grid signal. That is to say, it decreases in proportion to the increase in the impedance in the screen circuit.

It should be noted that the general idea of inserting an impedance in the screen circuit will work as well with any kind of screen grid tube as with a pentode tube. In other words, the presence of the third grid is not necessary to the functioning of the impedance in the screen circuit. Furthermore, an adjustable inductance in the screen circuit can be used for control of tone. The curves in Fig. 2 show the characteristics of a typical tone control arrangement applied to a pentode. The amplifier which was used in making these curves, was actually connected to a broadcast receiver and the operation of the tone control was observed.

In Fig. 3 there is shown another application of the principle embodied in Figs. 1 and 2. In this arrangement the output circuit of the pentode tube 1 is shown coupled by an audio frequency transformer T to the moving coil M of a dynamic speaker D. The screen grid '7 has connected in series therewith an anti-resonant network L, C. The curves in Fig. 4 graphically represent the relation between output volts and frequency variation in a pentode power tube employing an anti-resonant circuit in its screen grid circuit. Each of these "frequency-amplification curves is secured by employing a network of different suitable characteristics in the screen grid circuit. The reduction in amplification at 1500 cycles, as shown by the curves, is caused by the high impedance of the circuit L, C at its resonant frequency. By means of other networks having suitable characteristics, a desired amount of reduction of amplification may be introduced at selected frequencies.

In other words, there is shown in Fig. 3 an arrangement whereby the relation between the input to the loud speaker and the sound output thereof may be equalized. The three curves A, B and C in Fig. 5, represent three characteristics of the circuit shown in Fig. 3. Curve A in this figure represents a sound pressure curve of a typical moving coil loud speaker. (A moving coil loud speaker was used only as an example. Similar compensation could be applied to any other type of speaker, such as a magnetic speaker.) The high peak in this curve, which may be around 3,000 cycles, may be so high as to spoil the operation of the speaker. Curve B represents the amplification of the amplifier which drives the loud speaker. A resonant circuit similar to that used in making the curves in Fig. 4 is inserted in the screen circuit to produce the dip in the curve. The resonant circuit in the screen is tuned to the frequency of the loud speaker peak. Then, at this particular frequency, the power supplied to the loud speaker is reduced below the normal amount to compensate for the increased efficiency of the speaker. With this arrangement if a constant voltage is impressed on the input circuit of the amplifier and a sound pressure curve is made over a wide frequency range, the high peak at 3,000 cycles will not be found. Of course, it would not be possible to make a curve perfectly fiat, and, therefore, I have indicated in a conventional manner by curve C that some irregularity would remain. This loud speaker circuit is merely an illustration of one application of the pentode tube with a screen impedance. It could also be used to correct for variations in performance of other electrical devices at various frequencies.

It will be seen from Fig. 5 that the use of an anti-resonant network of suitable characteristic in the screen grid circuit of a pentode output tube results in a more satisfactory response curve when employing a pentode tube in the power stage of an audio frequency amplifier, and when employing a dynamic speaker as the load impedance. It is to be understood that while I have shown the present invention applied to a conventional power stage, the power stage will be used in the audio frequency amplifier of a radio receiver or any other type of electrical signalling arrangement wherein it is desired to employ a pentode tube as the power output tube.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and escribed, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, a pentode tube including a cathode, a control electrode, a screen grid, an anode, and a shield grid at cathode potential disposed between the screen grid and anode, a load in the external circuit connected to the anode whose impedance decreases with increasing frequency, and a reactive impedance in the screen grid circuit of the tube to counteract said load impedance decrease.

2. In combination, a pentode tube including a cathode, a control electrode, a screen grid, an anode, and a shield grid at cathode potential disposed between the screen grid and anode, a loud speaker in the external circuit connected to the anode, and a reactive impedance in the screen grid circuit of the tube.

3. In combination, a pentode tube including a cathode, a control electrode, a screen grid, an anode, and a shield grid at cathode potential disposed between the screen grid and anode, a loud speaker in the anode circuit and an inductive impedance in the screen grid circuit of the tube.

4. In combination, a pentode tube including a cathode, a control electrode, a screen grid, an anode, and a shield grid at cathode potential disposed between the screen grid and anode, a dynamic speaker in the anode circuit. and an inductance in the screen grid circuit of the tube.

5. In combination, a pentode tube including a cathode, a control electrode, a screen grid, an anode, and a shield grid at cathode potential disposed between the screen grid and anode, a dynamic speaker in the anode circuit, and an anti-resonant network in the screen grid circuit of the tube.

6. In combination, a pentode tube including a cathode, a control electrode, a screen grid, an anode, and a shield grid at cathode potential disposed between the screen grid and anode, a dynamic speaker in the anode circuit, and an adjustable inductance in the screen grid circuit of the tube.

7. In combination with an audio frequency power stage including a pentode tube having its output circuit coupled to an electrodynamic loud speaker, a reactive network in the screen grid circuit of the pentode tube, the network being designed to have a maximum impedance at a selected frequency and gradually diminishing impedance as the frequency is increased or decreased from the said selected frequency.

8. In combination with an amplifier tube provided with a screen grid electrode and having an input and output circuit, the output circuit including a loud speaker whose impedance varies r in a predetermined sense with the frequency of the energy applied to said input circuit over a frequency range, reactive means connected to the screen grid electrode of a value sufiicient for introducing into said output circuit an impedance varying in an opposite sense over said range to maintain the sound output of said speaker substantially constant over said range.

9. A method of controlling the frequency response of the output of an audio frequency amplifier stage including a tube of the screen grid type which consists in impressing on the input of the stage audio frequency energy to be amplified, and simultaneously varying the impedance of the screen grid circuit of the tube in such a manner that the said output has a decreasing high audio frequency response.

10. In combination, in an audio frequency amplifier, a tube including at least a cathode, a control electrode, a screen grid and an anode, a loud Loezisa speaker in the external circuit connected to the anode, said loud speaker having a resonance peak, and a reactive impedance in the screen grid circuit of the tube of a magnitude suificient to obtain a desired substantial reduction of said peak.

11. In combination, a tube including a cathode, a control grid, a screen grid and an anode, an electrodynamic loud speaker coupled to the anode of said tube, and an adjustable impedance in the screen grid circuit of the tube, said impedance being adjustable through a range of magnitudes suificient to vary the high frequency tone of the speakeix 12. In combination with an audio frequency power stage including a pentode tube having its output circuit coupled to an electrodynamic loud speaker having a characteristic impedance such that it possesses a pronounced resonant peak at a given audio frequency, a reactive network in the screen grid circuit of the pentode tube, the network being designed to have a maximum impedance at the given frequency and gradually diminishing impedance as the frequency is increased or decreased from the said given frequency whereby a frequency difference is produced at said frequency to compensate for the abnormally increased efficiency of the speaker at said given frequency.

RENE A. BRADEN. 

